Method for compensating secondary current of current transformers

ABSTRACT

A method for preventing the unwanted maloperation of a protective relay system that is caused by the imprecise detection of an actual secondary current value resulting from a measured secondary current being distorted comprises the steps of calculating difference values for sampled secondary currents; comparing absolute values of the difference values with a predetermined critical value, and determining a saturation starting moment of a current transformer when one of the absolute values exceeds the predetermined critical value; obtaining a magnetizing current at the saturation starting moment using the difference values if the saturation starting moment is determined, and obtaining a magnetic flux value in a steel core of the current transformer from a magnetization curve using the magnetizing current; calculating a magnetic flux value at a time after the saturation starting moment using a secondary current value measured at that time and the magnetic flux value obtained at the saturation starting moment so as to obtain a secondary current value consistent with a current transformation ratio at that time; obtaining a magnetizing current at that time from the magnetization curve using the magnetic flux value calculated at that time; and obtaining the secondary current value consistent with the current transformation ratio by adding the obtained magnetizing current and the measured secondary current value.

TECHNICAL FIELD

The present invention relates generally to a method of compensating thesecondary currents of current transformers in a protective relay systemfor protecting a power system, and, more particularly, to a method ofcompensating the secondary currents of current transformers, which canmake it possible to obtain precise secondary current values consistentwith a current transformation ratio even during the saturation of acurrent transformer so as to prevent the unwanted maloperation of aprotective relay system that is caused by the imprecise detection of anactual secondary current value resulting from a measured secondarycurrent being distorted because a current transformer is saturated by afault current.

BACKGROUND ART

A current transformer is an apparatus for measuring a current flowingthrough a power system and inputting the measured current to aprotective relay system. As shown in FIG. 1, the current transformercomprises a core 34 for concentrating a magnetic flux generated by aprimary current flowing through a line 32 and a secondary coil 36adapted to surround the core 34 for generating a secondary current usinga magnetic flux induced to the core 34. A current flowing through theline 32 functioning as a primary coil induces a current proportional tothe former current to the secondary coil 36, and the magnitude ofcurrent is determined according to a current transformation ratio. Inthis case, a steel core current transformer, in which the core 34 issteel, is chiefly used to maximize an interlinkage magnetic flux betweenthe line 32 and the secondary coil 36.

FIG. 2 illustrates a schematic equivalent circuit of a currenttransformer. In this drawing, L_(m) denotes a magnetizing inductance,i_(m) denotes a magnetizing current, i₁ denotes a secondary current(consistent with a current transformation ratio) induced to a secondaryside by a primary current, and i₂ denotes an actually measured secondarycurrent. The magnetizing inductance L_(m) is not a constant, but hasdifferent values depending upon magnetizing currents. In particular, ifa magnetic flux increases and exceeds a specific limit, the magnetizinginductance L_(m) varies considerably, which results from a variation inthe internal state of a current transformer. In such a case, it isstated that the current transformer is saturated.

Since the magnitude of the magnetizing current i_(m) is small duringnormal operations, the measured primary current value of the currenttransformer is proportional to the primary current value thereof, sothat a precise primary current value can be obtained from the measuredsecondary current value, thus causing no problem. However, if themagnetizing inductance value of the current transformer variesconsiderably by the saturation of the current transformer, the secondarycurrent value varies considerably. If this phenomenon is described basedon the equivalent circuit of FIG. 2, at the time of saturation, L_(m)value considerably decreases and, therefore, the magnetizing currenti_(m) increases, so that the difference between the actually measuredsecondary current i₂ and the secondary current i₁ consistent with thecurrent transformation ratio increases. Accordingly, a correlationbetween the actually measured secondary current i₂ and the secondarycurrent i₁ becomes different after saturation. Meanwhile, the currenttransformer detects the value of a current flowing through the lineusing i₂ even after saturation, so that it is imprecisely determinedthat the value of the current flowing through the line has decreased,thus causing the delay of the operation time of the protective relaysystem and the unwanted maloperation of the protective relay system.

FIGS. 6 a and 6 b are examples of magnetization curves showingcorrelations between magnetizing currents and interlinkage fluxes beforeand after saturation. FIG. 6 a shows the transition of a magnetizationcurve in unsaturated and saturated regions. The slope of themagnetization curve represents magnetizing inductances L_(m). FIG. 6 bshows an example of an actual magnetization curve. As shown in FIGS. 6 aand 6 b, magnetizing inductances are considerably different before andafter saturation.

For a conventional technology of compensating for current distortionresulting from the saturation of a current transformer, which is a mainreason for the unwanted maloperation of a protective relay system, andobtaining a secondary current consistent with an actual currenttransformation ratio, there is proposed a method of calculating amagnetic flux in a steel core constituting part of a current transformerat the time of saturation and compensating for the distorted secondarycurrent using the calculated magnetic flux to obtain a secondary currentconsistent with a current transformation ratio. However, theconventional method can be applied only to the case where a remanentmagnetic flux does not exist at an early stage. In the case where aremanent magnetic flux exists at an early stage, the application of theconventional method is limited, if the initial value of the remanentmagnetic flux is not known. In most applications, it is difficult tomeasure and estimate the value of the remanent magnetic flux usingexisting technology, so that the above-described disadvantage becomesfatal.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the prior art, and an object of the presentinvention is to provide a method for compensating the secondary currentsof current transformers, which can obtain a precise secondary currentvalue consistent with a current transformation ratio during thesaturation of a current transformer even when there is no information onthe value of an initial remanent magnetic flux, thus preventing theunwanted maloperation of a protective relay system caused by theimprecise detection of an actual current value resulting from thedistortion of the secondary current during the saturation of the currenttransformer.

In order to accomplish the above object, the present invention providesa method of compensating for secondary currents of current transformersdue to saturation, comprising the steps of calculating difference valueshaving at least second order for sampled secondary currents; comparingabsolute values of the difference values with a predetermined criticalvalue, and determining a saturation starting moment of a currenttransformer when one of the absolute values exceeds the predeterminedcritical value; obtaining a magnetizing current at the saturationstarting moment using the difference values if the saturation startingmoment is determined, and obtaining a magnetic flux value in a steelcore of the current transformer from a magnetization curve using themagnetizing current; calculating a magnetic flux value at a time afterthe saturation starting moment using a secondary current value measuredat that time and the magnetic flux value obtained at the saturationstarting moment so as to obtain a secondary current value consistentwith a current transformation ratio at that time; obtaining amagnetizing current at that time from the magnetization curve using themagnetic flux value calculated at that time; and obtaining the secondarycurrent value consistent with the current transformation ratio by addingthe obtained magnetizing current and the measured secondary currentvalue.

Preferably, the step of obtaining the magnetic flux value in the steelcore at the saturation starting moment may further comprise the step ofapproximating the magnetizing current at the saturation starting momentas a value obtained by assigning a negative sign to each differencevalue.

Preferably, the difference values may be second order difference values.

Preferably, the difference values may be third order difference values.

In accordance with another aspect of the present invention, the presentinvention provides a storage medium storing a computer program forcompensating for distortion of secondary currents of currenttransformers due to saturation, the program executing the steps ofcalculating difference values having at least second order for sampledsecondary currents; comparing absolute values of the difference valueswith a predetermined critical value, and determining a saturationstarting moment of a current transformer when one of the absolute valuesexceeds the predetermined critical value; obtaining a magnetizingcurrent at the saturation starting moment using the difference values ifthe saturation starting moment is determined, and obtaining a magneticflux value in a steel core of the current transformer from amagnetization curve using the magnetizing current; calculating amagnetic flux value at a time after the saturation starting moment usinga secondary current value measured at that time and the magnetic fluxvalue obtained at the saturation starting moment so as to obtain asecondary current value consistent with a current transformation ratioat that time; obtaining a magnetizing current at that time from themagnetization curve using the magnetic flux value calculated at thattime; and obtaining the secondary current value consistent with thecurrent transformation ratio by adding the obtained magnetizing currentand the measured secondary current value.

In accordance with still another aspect of the present invention, thepresent invention provides a protective relay system having a functionof compensating for distortion of secondary currents of currenttransformers due to saturation, comprising means for calculatingdifference values having at least second order for sampled secondarycurrents; means for comparing absolute values of the difference valueswith a predetermined critical value, and determining a saturationstarting moment of a current transformer when one of the absolute valuesexceeds the predetermined critical value; means for obtaining amagnetizing current at the saturation starting moment using thedifference values if the saturation starting moment is determined, andobtaining a magnetic flux value in a steel core of the currenttransformer from a magnetization curve using the magnetizing current;means for calculating a magnetic flux value at a time after thesaturation starting moment using a secondary current value measured atthat time and the magnetic flux value obtained at the saturationstarting moment so as to obtain a secondary current value consistentwith a current transformation ratio at that time; means for obtaining amagnetizing current at that time from the magnetization curve using themagnetic flux value calculated at that time; and means for obtaining thesecondary current value consistent with the current transformation ratioby adding the obtained magnetizing current and the measured secondarycurrent value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a general construction of a currenttransformer;

FIG. 2 is a diagram showing a schematic equivalent circuit of thecurrent transformer;

FIG. 3 is a graph representing the transition of secondary currentsbefore and after the saturation of the current transformer;

FIG. 4 is a flowchart showing an embodiment to which a compensationmethod of the present invention is applied;

FIG. 5 is a diagram showing an exemplary model system for verifying themethod of the present invention;

FIGS. 6 a and 6 b are graphs exemplifying magnetization curves used tocompensate the secondary currents of the current transformers;

FIG. 7 a is a graph showing primary currents divided by a turn ratio(that is, secondary currents consistent with a current transformationratio) and measured secondary currents;

FIG. 7 b is a graph showing magnetic fluxes calculated in the case of+80% remanent magnetic flux;

FIG. 7 c is a graph showing magnetizing currents estimated in the caseof +80% remanent magnetic flux;

FIG. 7 d is a graph showing the secondary currents of the currenttransformer before and after compensation; and

FIG. 7 e is a graph showing the transient errors of the estimatedsecondary currents of the current transformer.

Description of Reference Numerals of Principal Elements of the Drawings

32: line

34: steel core

36: secondary winding

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the present invention is described below indetail with reference to the accompanying drawings.

FIG. 3 exemplarily shows the transition of secondary currents i₂[n](solid line) actually measured in the vicinity of the saturationstarting point of a current transformer. The current transformer issaturated at the time n=m. Currents are measured after saturation. Inthis case, secondary currents measured before saturation are representedby i₂₁[n] and secondary currents measured after saturation arerepresented by i₂₂[n]. Since secondary currents i₁[n] consistent with acurrent transformation ratio are actually measured secondary currents,that is, i₂[n] measured before saturation, the actually measuredsecondary currents substantially consist with the i₂₂[n] in the drawing,which reflect the secondary currents i₁[n] consistent with the currenttransformation ratio. After saturation, a secondary currents i₁[n]consistent with a current transformation ratio are considerablydifferent from actually measured secondary currents, as shown in. FIG.3. In FIG. 3, secondary currents consistent with the currenttransformation ratio after saturation are represented by a dotted lineextending from i₂₁[n].

Secondary currents i₁(t) precisely reflecting primary currents at thetime of a fault and consistent with a current transformation ratio canbe expressed by the following equation:

$\begin{matrix}{{i_{1}(t)} = \begin{bmatrix}{{I_{\max}\left\lbrack {{\cos\left( {{\omega\; t} - \theta} \right)} - {{\mathbb{e}}^{{- t}/T_{p}}\cos\;\theta}} \right\rbrack},} & {{{for}\mspace{14mu} t} \geq 0} \\{0,} & {{{for}\mspace{14mu} t} < 0}\end{bmatrix}} & (1)\end{matrix}$

where I_(max), T_(p) and θ denote a maximum fault current, a primarytime constant and a fault inception phase angle, respectively. In thiscase, actually measured secondary currents are expressed as follows:i ₂ [t]=Ae ^(−t/T) ^(s) +Be ^(−t/T) ^(p) −C sin(ωt−θ−φ)  (2)

where T_(s) denotes a secondary time constant and tan φ=T_(s).

The discrete time version i₂[n] of the secondary currents can beobtained by the following equation:

$\begin{matrix}{{i_{2}\lbrack n\rbrack} = {{A\;{\mathbb{e}}^{{- n}\;{T/T_{s}}}} + {B\;{\mathbb{e}}^{{- n}\;{T/T_{p}}}} - {C\mspace{11mu}{\sin\left( {{\frac{2\;\pi}{N}n} - \theta - \varphi} \right)}}}} & (3)\end{matrix}$

where T denotes a sampling interval and N denotes the number of samplesper cycle. This equation comprises two exponential terms, which decreaseexponentially, and one sinusoidal term.

The first order difference of i₂[n] is expressed as follows:

$\begin{matrix}\begin{matrix}{{{del1}\lbrack n\rbrack} = {{i_{2}\lbrack n\rbrack} - {i_{2}\left\lbrack {n - 1} \right\rbrack}}} \\{= {{{A\left( {1 - {\mathbb{e}}^{T/T_{s}}} \right)}\;{\mathbb{e}}^{{- n}\;{T/T_{s}}}} + {{B\left( {1 - {\mathbb{e}}^{T/T_{p}}} \right)}\;{\mathbb{e}}^{{- n}\;{T/T_{p}}}} -}} \\{{C\left( {2\mspace{11mu}\sin\;\frac{\pi}{N}} \right)}\;{\sin\left( {{\frac{2\;\pi}{N}n} - \theta - \varphi - \frac{\pi}{N} + \frac{\pi}{2}} \right)}}\end{matrix} & (4)\end{matrix}$

If a frequency is 60 Hz and N=64, T=0.26 ms. If Ts=1 s and Tp=0.02 s,the reduction ratios of the two exponential terms, that is, 1−e^(T/T)^(s) and 1−e^(T/T) ^(p) , are 0.00026 and 0.0131, respectively.Accordingly, if the time constant is sufficiently large, the exponentialterms of del1[n] are negligible.

The magnitude of the sinusoidal term of del1[n] is 2 sin(π/N)C=0.098C.That is, about 10% of the components of the sinusoidal term of del1[n]remain. Consequently, if the secondary currents have the form ofEquation (3), del1[n] has almost no exponential terms but the sinusoidalterm, and the magnitude of Equation (3) is 10% of an original magnitude.

The second order difference function of i₁[n] is defined as follows:del2[n]=del1[n]−del1[n−1]  (5)

In this equation, if N=64, it can be appreciated from Equation (4) thatthe magnitude of del2[n] is [2 sin(π/N)]²C=0.009604C, that is, 1% of themagnitude of the sinusoidal term of i₂[n].

It is assumed that a current is measured at n=m+1 after saturation. Ifi₂₁[n] represents currents before saturation and i₂₂[n] representscurrents after saturation, as defined above, Equation (3) is formed.Further, i₂₁[m]=i₂₂[m] and i₂₁[m+1]≠i₂₂[m+1]. i₁[m+1], which aresecondary currents precisely reflecting primary currents to obtain andconsisting with a current transformation ratio, can be approximated asi₂₁[m+1] extending from the transition of secondary currents measuredbefore saturation, which correspond to values on an extension linerepresented by a dotted line, as shown in FIG. 3.

If [2 sin(π/N)]²C is considerably small, del2[n] can be used todetermine a saturation starting moment. For example, C=100 A and N=64,[2 sin(π/N)]²C=0.96 A. del2[n] is a sinusoidal wave in the range of n≦m(range before saturation), and the magnitude thereof does not exceed0.96 A. Further, at the saturation starting point n=m+1, del2[m+1] isexpressed as follows:

$\begin{matrix}\begin{matrix}{{{del2}\left\lbrack {m + 1} \right\rbrack} = {{i_{22}\left\lbrack {m + 1} \right\rbrack} - {2{i_{22}\lbrack m\rbrack}} + {i_{21}\left\lbrack {m - 1} \right\rbrack}}} \\{= {{i_{22}\left\lbrack {m + 1} \right\rbrack} - {2{i_{21}\lbrack m\rbrack}} + {i_{21}\left\lbrack {m - 1} \right\rbrack}}}\end{matrix} & (6)\end{matrix}$

The approximation of Equation (6) can be made using the relation of thefollowing Equation (7). Equation (7) corresponds to the second orderdifferences of secondary currents in the range before saturation. Sincethe second order differences are considerably small (0.96 A), asdescribed above, which is negligible. The maximum error of theapproximation is less than 0.96 A.i ₂₁ [m+1]−2i ₂₁ [m]+i ₂₁ [m−1]≈0  (7)

Accordingly, del2[m+1] can be approximated as i₂₂[m+1]−i₂₁[m+1]. In thiscase, since i₂₁[m+1] is a point on a virtually extended line, i₂₁[m+1]can be considered a secondary current value measured at n=m+1 if it isassumed that a transition before saturation continues. Accordingly,del2[m+1] is a magnetizing current at the moment, that is, a valueobtained by adding a negative sign to a current flowing through amagnetizing branch of FIG. 2.

Accordingly, if −del2[m+1] is substituted to the magnetization curve, amagnetic flux can be determined at a saturation starting moment.Thereafter, secondary currents applying with a current transformationratio can be estimated by calculating the magnetic flux of a steel coreat every moment using the magnetic flux value at the saturation startingmoment obtained above as an initial value, obtaining a magnetizingcurrent at a corresponding moment by substituting the magnetic flux ofthe steel core to the magnetization curve, and adding a measuredsecondary current to the magnetizing current.

FIG. 4 is a flowchart showing a method of determining a saturationstarting moment using the second order difference for secondary currentsmeasured as described above, obtaining secondary currents consistentwith a current transformation ratio and compensating the measuredsecondary currents, in accordance with an embodiment of the presentinvention.

Sat₁₃ ind, which is an index indicating whether or not saturation isestablished, is defined at step S10, and an initial value is set. Forexample, the case where sat_ind is 0 is defined as saturation not beingestablished, while the case where sat_ind is 1 is defined as saturationbeing established. Thereafter, a secondary current sampled at acorresponding moment (for example, n=k) is received at step S30. Ifsat_ind=0, that is, before saturation, a second order difference valueis calculated at step S40. If the absolute value of the second orderdifference is more than a predetermined critical value, it is determinedthat saturation is established at that moment at step S50. If it isdetermined that saturation is established at that moment, the magneticflux value at the saturation starting moment is estimated bysubstituting −del2[m+1] to a magnetization curve using the methoddescribed above, and sat_ind, which is the index indication thatsaturation is established, is set to “1” and displayed at step S60.Thereafter, the measured secondary current value is compensated toobtain a secondary current value consistent with a currenttransformation ratio at step S70. The compensated value is transmittedto a current transformer protection algorithm at step S80. If saturationis not established, the measured secondary current value is a secondarycurrent value consistent with the current transformation ratio andreflects the transition of currents as it is, so that the measuredsecondary value is directly transmitted to the current transformerprotection algorithm.

Since sat_ind has been set to “1” at the next moment (for example,n=k+1), the step S20 of inputting a current sampled at a correspondingmoment is performed. The magnetic flux of the steel core is calculatedusing the magnetic flux at the saturation starting moment obtaineddescribed above at step S32. A magnetizing current is calculated fromthe magnetization curve at step S34. A secondary current value isobtained by compensating the measured current value using the calculatedmagnetizing current at step S70. The compensated current value istransmitted to the current transformer protection algorithm at step S80.

Although in the above description, the case of estimating the magneticflux value at the saturation starting moment using the second orderdifference has been chiefly discussed, a secondary current consistentwith a current transformation ratio may be estimated by obtaining amagnetizing flux value at a saturation starting moment using third orhigher order difference values.

The case where the magnetic flux at the saturation starting moment isestimated using the third order difference value is described below.

The third order difference function del3[n] of the measured secondarycurrent i₂[n] is defined as follows:del3[n]=del2[n]−del2[n−1]  (8)

In the equation, del3[n] is used to determine whether saturation isestablished, the principle of which is as follows. It can be appreciatedfrom Equations (4) and (5) that del3[n] comprises a sinusoidal term [2sin(π/N)]³C=0.000941C.

If [2 sin(π/N)]³C is considerably small, del3[n] can be used todetermine the saturation starting moment. For example, if C=100 A andN=64, [2 sin(π/N)]³C=0.09 A.

Accordingly, del3[n] is a sine wave in the range of n≦m (range beforesaturation), and the magnitude of del3[n] does not exceed 0.09 A.Further, at the saturation starting moment n=m+1,del3[m+1]=i ₂₂ [m+1]−3i ₂₁ [m]+3i ₂₁ [m−1]−i ₂₁ [m−1]  (9)

Equation (9) can be estimated to the following Equation (10), themaximum error of which is less than 0.09 A.i ₂₁ [m+1]−3i ₂₁ [m]+3i ₂₁ [m−1]−i ₂₁ [m−1]≈0  (10)

Accordingly, del3[n] can be estimated to i₂₂[m+1]−i₂₁[m+1]. In thiscase, i₂₁[m+1] is a point on a virtually extended line, which can beconsidered a secondary current value at the moment of n=m+1 if it isassumed that a current transformer is not saturated. Accordingly,i₂₂[m+1]−i₂₁[m+1] is a magnetizing current at the moment, that is, avalue obtained by adding a negative sign to a current flowing throughthe magnetizing branch of FIG. 2 at the moment.

By substituting del3[n] to a magnetization curve, a magnetizing flux atthe saturation starting moment can be known. Accordingly, from the nextmoment, by calculating the magnetic flux of a steel core at everymoment, obtaining a magnetizing current by substituting the calculatedmagnetic flux to the magnetization curve and adding a secondary currentto the magnetizing current, a secondary current consistent with acurrent transformation ratio can be estimated.

This case can be represented by the method of FIG. 4. The case can beimplemented according to the same flowchart if the absolute values ofsecond order differences are change to the absolute values of secondorder differences. It is not necessary to use the same determinationfunction to perform the determination of the saturation starting moment(S50) and the estimation of a magnetic flux value at the saturationstarting moment (S60). There can be various modifications, such asmodifications in which the determination of the saturation startingmoment is performed using the second order difference and the estimationof a magnetic flux value is performed using the third order differenceand vice versa.

To verify the method of the present invention, a model system of FIG. 5is selected. The magnetization curve of FIG. 6 b is used to compensatethe secondary current of the current transformer of FIG. 5. Themagnetizing flux of the saturation point of the current transformer is1.512 Vs.

At the time of an A phase ground fault spaced apart from a P bus by 2 kmin the system of FIG. 5, results obtained by applying the method of thepresent invention to the case where 80% remanent magnetic flux of thecurrent transformer saturation point, that is, 1.2 Vs, exist in thesteel core of the current transformer are shown in FIGS. 7 a to 7 e. InFIG. 7 a, a solid line represents currents (secondary currentsconsistent with a current transformation ratio) obtained by dividingprimary currents by a turn ratio, and a dotted line represents measuredsecondary currents. It can be appreciated from FIG. 7 a that secondarycurrents, measured while a current transformer is saturated by a largeremanent magnetic flux, are severely distorted.

It is determined using the second order differences of the secondarycurrents whether saturation is established. Magnetic fluxes calculatedafter saturation are represented in FIG. 7 b.

Magnetic currents are estimated by substituting these magnetic fluxes toa magnetization curve, and the estimated magnetic fluxes are shown inFIG. 7 c.

Secondary currents consistent with a current transformation ratio areestimated by adding measured secondary currents to the estimatedmagnetizing currents, and the estimated secondary currents are shown inFIG. 7 d. In FIG. 7 d, a dotted line represents measured secondarycurrents, and a solid line represents secondary currents consistent witha current transformation ratio and estimated by the method of thepresent invention.

Transient errors calculated to verify the precision of the estimatedsecondary currents of FIG. 7 d are shown in FIG. 7 e. Since the error ofa current transformation ratio is considerably small, it can beappreciated that the proposed method can estimate precise secondarycurrents consistent with the current transformation ratio from distortedsecondary currents.

Even in the case where a large remanent magnetic flux exist, secondarycurrents distorted by the saturation of a current transformer can becompensated to obtain secondary currents consistent with a currenttransformation ratio.

As described above, although in the detailed description of the presentinvention, the present invention has been described in conjunction withconcrete embodiments, these embodiments should be consideredillustrative ones. Of course, various modifications are possible withoutdeparting from the scope and spirit of the invention. Accordingly, thescope of the present invention must not be determined only by thedescribed embodiments, but must be determined by the equivalent ofclaims as well as the claims.

INDUSTRIAL APPLICABILITY

In accordance with the present invention, it is possible to estimate aprecise secondary current consistent with a current transformation ratiofrom a measured secondary current even during the saturation of acurrent transformer, thus preventing the unwanted maloperation of aprotective relay system at a fault and enabling the rapid and preciseprotection of a power system.

1. A method of compensating for secondary currents of currenttransformers due to saturation, comprising the steps of: (a) determininga saturation starting moment of a current transformer by calculatingdifference values having at least second order for sampled secondarycurrents and comparing absolute values of the difference values with apredetermined critical value; (b) obtaining a secondary current valueconsistent with a current transformation ratio where the effects of thesaturation are removed, at a time after the saturation starting momentby: (i) obtaining a magnetizing current at the saturation startingmoment using the difference values if the saturation starting moment isdetermined, and obtaining a magnetic flux value in a steel core of thecurrent transformer from a magnetization curve using the magnetizingcurrent; (ii) calculating a magnetic flux value at a time after thesaturation starting moment using a secondary current value measured atthat time and the magnetic flux value obtained at the saturationstarting moment; (iii) obtaining a magnetizing current at that time fromthe magnetization curve using the magnetic flux value calculated at thattime; and (iv) adding the obtained magnetizing current to the measuredsecondary current value at that time.
 2. The method as set forth inclaim 1, wherein the step of obtaining the magnetic flux value in thesteel core at the saturation starting moment further comprises the stepof approximating the magnetizing current at the saturation startingmoment as a value obtained by assigning a negative sign to eachdifference value.
 3. The method as set forth in claim 1, wherein thedifference values are second order difference values.
 4. The method asset forth in claim 1, wherein the difference values are third orderdifference values.
 5. The method as set forth in claim 2, wherein thedifference values are second order difference values.
 6. The method asset forth in claim 2, wherein the difference values are third orderdifference values.
 7. A storage medium storing a computer program forcompensating for distortion of secondary currents of currenttransformers due to saturation, the program executing the steps of: (a)determining a saturation starting moment of a current transformer bycalculating difference values having at least second order for sampledsecondary currents and comparing absolute values of the differencevalues with a predetermined critical value; (b) obtaining a secondarycurrent value consistent with a current transformation ratio where theeffects of the saturation are removed, at a time after the saturationstarting moment by: (i) obtaining a magnetizing current at thesaturation starting moment using the difference values if the saturationstarting moment is determined, and obtaining a magnetic flux value in asteel core of the current transformer from a magnetization curve usingthe magnetizing current; (ii) calculating a magnetic flux value at atime after the saturation starting moment using a secondary currentvalue measured at that time and the magnetic flux value obtained at thesaturation starting moment; (iii) obtaining a magnetizing current atthat time from the magnetization curve using the magnetic flux valuecalculated at that time; and (iv) adding the obtained magnetizingcurrent to the measured secondary current value at that time.
 8. Thestorage medium as set forth in claim 7, wherein the step of obtainingthe magnetic flux value in the steel core at the saturation startingmoment further comprises the step of approximating the magnetizingcurrent at the saturation starting moment as a value obtained byassigning a negative sign to each difference value.
 9. The storagemedium as set forth in claim 7, wherein the difference values are secondorder difference values.
 10. The storage medium as set forth in claim 7,wherein the difference values are third order difference values.
 11. Thestorage medium as set forth in claim 8, wherein the difference valuesare second order difference values.
 12. The storage medium as set forthin claim 8, wherein the difference values are third order differencevalues.
 13. A protective relay system having a function of compensatingfor distortion of secondary currents of current transformers due tosaturation, comprising: (a) means for determining a saturation startingmoment of a current transformer by calculating difference values havingat least second order for sampled secondary currents and comparingabsolute values of the difference values with a predetermined criticalvalue; (b) means for obtaining a secondary current value consistent witha current transformation ratio where the effects of the saturation areremoved, at a time after the saturation starting moment by: (i)obtaining a magnetizing current at the saturation starting moment usingthe difference values if the saturation starting moment is determined,and obtaining a magnetic flux value in a steel core of the currenttransformer from a magnetization curve using the magnetizing current;(ii) calculating a magnetic flux value at a time after the saturationstarting moment using a secondary current value measured at that timeand the magnetic flux value obtained at the saturation starting; (iii)obtaining a magnetizing current at that time from the magnetizationcurve using the magnetic flux value calculated at that time; and (iv)adding the obtained magnetizing current to the measured secondarycurrent value at that time.
 14. The protective relay system as set forthin claim 13, wherein the step of obtaining the magnetic flux value inthe steel core at the saturation starting moment further comprises thestep of approximating the magnetizing current at the saturation startingmoment as a value obtained by assigning a negative sign to eachdifference value.
 15. The protective relay system as set forth in claim13, wherein the difference values are second order difference values.16. The protective relay system as set forth in claim 13, wherein thedifference values are third order difference values.
 17. The protectiverelay system as set forth in claim 14, wherein the difference values aresecond order difference values.
 18. The protective relay system as setforth in claim 14, wherein the difference values are third orderdifference values.